A limiting factor in improving the performance of a turbomachine is often the rotational speed at which the impellers of the machine can operate. The stress levels developed in the impellers often prohibit operation at higher speeds which would provide greater performance. Structural considerations often run counter to aerodynamic considerations in the design of impellers. Advanced aerodynamic features such as thin blades, blade shrouds, backward blade curvature, and reduced impeller weight all incur higher operating stresses than more conservative features and therefore tend to reduce the possible operation speed. The costs associated with introducing such advanced features also are high, and suitable materials and methods of manufacturing are limited. Thus it is desirable to be able to reduce operating stress levels in such impellers to allow their operation at higher rotational speeds.
During operation of a turbomachine impeller, stresses occur and vary continuously throughout the impeller, being a combination of primary and secondary stresses created by the applied forces and the impeller's configuration. Primary stresses are developed by imposed loadings on the impeller, such as the centrifugal force produced by rotation of the impeller. A basic characteristic of a primary stress is that it is not self limiting. Primary stresses which considerably exceed the yield strength of the impeller material cause gross distortion or rupture of the impeller, which shall be termed failure of the impeller.
Secondary stresses are developed in the impeller by the constraints imposed by adjacent parts or by the impeller itself, that is, by self constraint. A basic characteristic of a secondary stress is that it is self limiting. Local yielding and distortions can occur as a result of secondary stresses, but failure does not usually occur from secondary stresses. Residual stresses, by their nature, are secondary stresses which can be developed through the application of both primary and secondary stresses to the impeller.
In operation, vibratory stresses are also produced by the dynamic environment of the impeller and are superimposed on the steady stresses. Vibratory stresses can quickly cause fatigue fracture of the impeller.
As used herein, "residual stress" shall mean internal stress existing in a material with no external forces applied, developed by the material itself, that is, by self constraint in the material.
As used herein, "compressive stress" shall mean a stress which causes a material to shorten in the direction of the applied force producing the stress.
As used herein, "tensile stress" shall mean a stress which causes a material to lengthen in the direction of the applied force producing the stress.
As used herein, "steady stress" shall mean a stress that does not vary with time if all external forces are steady, that is, do not vary with time, as distinguished from alternating or vibratory stress.
As used herein, "yielding" shall mean plastic deformation or permanent change in shape or size of a material, without fracture, resulting from the application of a stress.
As used herein, "tolerable yielding" shall mean yielding only to an extent which does not render an object unsuitable for further functioning intended for the object, such as yielding which does not change the shape or size or balance of an object so as to render it unsuitable for further functioning as intended.
This invention may be applied to any structure or device in which the applied loading creates a distributed primary stress field such that there are localized regions of high primary and secondary stress uncoupled from each other, uncoupled in the sense that they do not share a common geometric constraint. In addition the structure or device must be of a material which has adequate ductility to permit reasonable yielding or plastic deformation without fear of failure. A typical metal turbomachine impeller is such a structure.
The steady stresses occurring in a typical metal turbomachine impeller during operation may be computed by known methods such as finite element analysis. Steady stresses are produced by centrifugal forces due to rotation of the impeller, temperature differences between different regions of the impeller, and dynamic pressure forces imposed by fluids contacting the impeller.
In rotational operation of an impeller, peak stresses occur at various locations in the impeller. Increasing the capability of just these specific locations to withstand stress increases the operating capability of the impeller. A method for improving the capability of a specific location to withstand stress during rotation is to induce a beneficial residual stress at the location. Since the peak stresses are usually tensile, inducing a residual compressive stress is usually beneficial. A method for inducing a residual compressive stress at a specific selected location is to overstress the location so that local yielding occurs at the location. Upon relieving the momentary overstress, the unyielded material surrounding the yielded material exerts a residual compressive stress upon the yielded material. This can be accomplished in an impeller at a location experiencing the highest steady tensile stress by rotating the impeller to a peak speed higher than the design speed so as to develop a tensile stress which induces tolerable local yielding at this location.
In an impeller, a location particularly subject to the development of vibratory stress and thus fatigue failure is the location where the longest blade length occurs, termed the eye of the impeller. Thus this is a location where it is often desirable to induce residual compressive stresses which will lower the steady tensile stress occurring in rotation, thereby increasing the capability of this location for vibratory stress. However, the eye location is not usually the location where the highest tensile stress occurs during rotation. Other locations in the impeller usually experience higher steady tensile stresses during rotation than the eye location. If an attempt is made to introduce a residual compressive stress immediately at the eye location by causing local yielding at the eye location, excessive yielding may occur at other locations in the impeller experiencing higher steady tensile stresses such as to render the impeller useless for service.
The object of the present invention is to provide a method for improving the operating stress capability of a body to be subjected to rotation.
It is a feature of this invention that the operating stress capability of a body in rotation is improved by introducing beneficial residual stresses at a selected location in the body having high local stress levels.
It is another feature of this invention that the operating stress capability of a body in rotation is improved by inducing tolerable local yielding at locations in the body having high local stress levels.
It is an advantage of this invention that the operating stress capability of a body in rotation is improved simply by a series of successive rotations at selected peak speeds higher than the design speed.
It is an advantage of this invention that the operating stress capability of a selected location in a body can be improved when the selected location is not the location experiencing the highest local steady tensile stress in the body.